Ceramic substance and method of making



March 15, 1960 s. c. LYoNs CERAMIC SUBSTANCE AND METHOD OF MAKING FiledMarch 22, 1956 United States Patentl` O i 'CERAMIC SUBSTANCE AND METHODF MAKING Sanford C. Lyons, Bennington, Vt., assigner to Georgia KaolinCompany, Elizabeth, NJ., a corporation of New Jersey This application isa continuaton-in-part of my cepending application Serial No. 479,377,filed January 3, 1955 and later abandoned. The invention relates to acontrollable and reproducible ceramic substance and a method of makingthe same. Heretofore'the substances of which ceramic articles have beenmade have been mainly clays of the kaolinite type with or without theadmixture of other ingredients. Since the natural deposits of clay varyconsiderably in composition and since there is liable to be variation inthe composition in the clay taken from a single locality, thepreparation of a substance to be made into ceramic articles has beenguess Work to a considerable extent.

The ceramist, from the dawn of history, has been of necessity anempiricist. As basic raw material, clay has been of such extremely neparticle-size that, literally, until within the last two decades noinstruments were available whereby he could determine clearly the size,shape, or other basic features of these particles. Not only were claysvery finely divided, but their particles have never been known to occurin nature in monodispersion, i.e., within a single narrow particle-sizerange. They are always variant in size, ranging from roughly 0.002" (50microns) down to less than l/ 1,250,000 diameter (0.02 micron), so thatany measure of iineness control seemed impracticable.

Some idea of the complexities involved in this matter of particle-sizedistribution can be gathered from computations which have shown that ina single gram of clay consisting of particles having a diameter of 0.1micron or less there will be near 1015 such articles, (See Table IV inSome Ceramic Mechanisms and New Materials by Koenig and Lyons in theApril 1955 issue of Ceramic Age, page 26.)

Making the problem even more confusing, and only recently discovered, isthe fact that certain clay particles suddenly change their shape atabout two microns equivalent spherical diameter and each of the twodifferently shaped particles exert divergent effects upon the ceramicprocess and product. Since the early ceramist had no hint of thesefacts, and, furthermore, no control over them had he known about them,he literally had to proceed by guesswork. Consequently, there has been ahigh percentage of weak or defective articles by reason of theexperimentation required to obtain y'satisfactory results. The troublesarise chiefly from dimensional changes occurring in tiring and coolingthe articles. The cause and control of such changes have not been clear-1y understood until recent developments in clay technology have not onlymade possible the segregation of the components of a clay but havethrown considerable light on the behavior of the individual componentsin the tiring and cooling operations. In an effort to control thedimensional changes in ceramic substances, other cornponents such asfinely ground flint have been added to the clay. L

Ground int (quartz) undergoes a sharp dimensional 5 very slowly at/thisapproximately 600 C. temperaturel i 2,928,751 Patented Mar. 15,4 196,0

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600 C. At this temperature, many of the clay particles have alreadyassumed their positions of initial set and as long as ground ilint isused in the body in an effort to reduce shrinkage, the firing of theware must be done in order to give the dimensional stresses, caused bythe int phase, an opportunity to relieve themselves throughout the mass.This effect is observed both as the ware is heated up and also as itcools. Not only doesv this result in a slowing down of the iiringprocess with consequent fuel consumption and of course a decrease inoutput per kiln, but it poses a constant danger to the structuralquality of the ware itself. Consequently, if a non-shrinking componentwere available which did not have the objectionable dimensional change,it would be a valuable raw material for the ceramist. Such a material isdescribed hereinbelow.

An object of the present invention is to provide an improved ceramicsubstance which is accurately reproducible so that uniform quality ofproducts can be maintained, which results in ceramic istructures ofreduced shrinkage, increased strength and greater dimensional precision,and which can be tired faster, thus saving time and fuel. A method bywhich a substance of the foregoing character is obtained comprisessegregating certain of the constituents, and then mixingtogetherpredetermined percentages of the constituents with or without anyadditional ingredients which may be desired. This method has beenevolved as a result of recent research and experimentation withclays ofdifferent types. Examination of kaolinite clay under the highmagnification made possible by the electron microscope has revealed thatparticle size plays a vital part in the behavior of the clay. There hadbeen previous experimental evidence of a sharp diierence between clayparticles greater than 2 microns and particles smaller than 2 microns(equivalent spherical diameter) as respects their gloss-developingproperties and their hiding power in paper coating, this discovery beingdescribed in the U.S.y Patent to o Maloney No. 2,158,987. More recentexperimental change known as the quartz inversion poin at abouttechniques, including the use of electron microscopes, have demonstratedthat kaolinitic particles of less than two microns equivalent sphericaldiameter are in the form of dat hexagonal crystal plates. Kaoliniteparticles having a size of two microns or more are stacks of such plateshaving face-to-face adherence. The plates vary somewhat in form. Inclays suitable for paper-coating, relatively thick plates with fairlyregular hexagonal contours predominate. In ceramic-type kaolins,including ball clay, the plates are generally thinner with ragged edges.The stacks also vary. In some clays the plates composing the stacks aregenerally arranged one above the other in a regular pile. In other claysthe plates in most of the stacks are more or less irregularly arranged.

In view of the irregular shapes of most clay particles, the termdiameter as applied herein to such particles is understood to meanequivalent spherical diameter, that is, the diameter of a sphere havingthe same sedimentation rate in water.

It has recently been clearly shown that lthere is a criticality aboutthe dimensional point at which kaolinite other hand, the longvermiculite-type stacked agglomerates are seldom seen smaller than 2microns diameter.

It shouldbe borne in mind that there is an equall criticality in theceramic properties of these two differ-'I ent types of kaolinateparticles. i 1 l The property of plasticity (with its concomitantproperties of dry-strength and tired-strength) is, to a truly surprisingdegree, contributed by the plate-like tine particles. On the other hand,unfortunately, these same fine particles contribute high drying and.,tiring, shrinkage. Notonly is this feature most embarsassingandtroublesome to the ceramist because it renders his control ofthedimensional precision of his finished wares diicult andv uncertain, butmore especially is this shrinkage feature undesirable in that it causesthe development of strainsV within the ceramic body which nally resultin minute shrinkage cracks which destroy the structural solidity of theware and thereby greatly reduce its finished strength and/or utility. Y

. In tiring ceramic articles, observed expansion and contraction of thearticles has given rise to speculation as to dimensional changes in theindividual particles. modern microscopic studies have indicated thatkaolnite stacks when heated to about 600ar or over, expand lengthwise(accordion-fashion) with an increase of length of about 20%, and thatthis expansion appears to be irreversible. If such particles or stacksare heated up to 1000 C. or so, very little agglomeration of stacksoccurs. At similar temperatures, however, there is extensive agglomeration of the colloidal particles (kaolinte plates).- Hence it isevidently the colloidal and/or plate shaped particles in a ceramic claywhich provide the cohesive strength of the article. it is also'probablethat the dissimilar dimensional changes in the stacks and plates whenfired set up internal stresses' which mayY result in cracks and renderthe article fragile. Aurobject of-the,`

present.- invention is to reduce dimensionalchanges incident to firingby employingpre-calcined stacks in theV ceramic material to be fired,such material being amixture of definite ingredients in reproducibleproportions.

This makes possible a high degree of uniformity in the ceramic product,both in dimensions and physical propcities, and makes possible a fastertiring process which results in a saving of time and fuel.

From the foregoing, it is clear that vboth types "of par-Y ticles arerequired in ceramic bodies. The tine plate particles are required forstrength while the coarser stacklike particles are required for theirnonshrinkage contribution. It should be clear also that for ydiiferenttypes of'ware and for diiierent sizes of ware, ktheloptimurn proportionof plates (strength-giving) to stacks (nonshrinking) will differ.Seldom, if. ever, areA natural clays fou-nd with such optimumproportions' of plates toAV stacks. `In the few cases where such `mayhave existed, there has been no assurance whatever of continuityoflsupply and moreover, there has been no standard way to correct orcontrol deviation therefrom.

In the present process, .the naturalclay is first segregated into itstwo differently shaped crystal particle components, viz., plates andstacks.

This is achieved by blunging crude clay into deiiocculated aqueoussuspension and effecting a sharp classication into two separatefractions at about 2 micronsV Details of a process particularlyeffective for diameter. this purpose are described in my U.S. PatentsNos. 2,085,537, 2,085,538 and 2,179,154. The tine-particles (plates) arekept separate. The coarse particles (stacks) are dried and calcined to atemperature above 600"- C. and preferably 1000" C. These are thenblended `.back

with the above mentioned fine particles (plates) insuch` proportions asare found to give the! optimum properties of strength, shrinkage, etc.,required for the particular purpose at hand. In addition to the originalclassiiication of the particles into groups consisting respectively ofparticles over and under 2 microns equivalent sphericaidiameter, afurther classification may be made of the coarser, particles. Forexample, it maybe advantageous` forA some product to exclude' particlesoriginally greater .than l microns. .In such-.caserthe tirstfstep ofclassication would befollowedlby' afsecondffstep #to Y powder or acohesive mass, and it is so used herein.

Thus by calcined clays is meant clays which have been strongly heated.

lays of fortuitous and/or indiscriminate particle-size have in thepastbeen calcined and then reground for use in ceramic bodies. However,these prior art calcined clays have embodied certain inferioritiesand/or severe production diiculties which the instant inventionobviates. So far as I am aware, no one has heretofore realized that bytaking advantage of the removal of particles finer than about 2 micronsit is possible to obtain kaolinite clayswhich can be calcined to acondition of being substantially self-pulverized after calcining. Clayscalcined bythe usual methods issue from the kilns in a form reminiscentof fragments of fire brick. This means that in order to make the latterusable in high grade ceramic wares, it is necessary to grind them. Asall persons familiar with the art of grinding well know that it is notparticularly difficult to grind fire brick to ordinary dimensions, e.g.40 to 80 mesh, but when it must be ground to ceramic dimensions, that iswith most than 2 microns before calcining obviates thisdifliculty,

well-known in the Yvart.

a new production field is open to the ceramist.

It has been mentioned that by first separating the natural clay into itstwo component type crystals, and then recombining these components invarious ratios, improved ceramic structures have been obtained byutilizing proportions of plates to stacks differing from those found innature. A surprising and valuable further discovery was made whencombinations of uncalcined plates'with the separately calcined stackswere made. For reasons as yet not clear, many of the mixtures whichcontained calcined stacks instead of the natural uncalcined stacks gavetired strength far in excess of those obtained with any combinationswhich contained only the uncalcined stacks. This fact is considered tobe one of the major discoveries in connection with this invention.

The drawing shows a diagrammatic layout of wellknown pieces of apparatuswhich can be employed .-in producing my improved ceramic material.

A blunger 10 is employed to make a uid slurry of kaolinite clay with asuitable dispersing chemical, such as sodium silicate Yor. sodiumphosphate, in a manner The slurry is passed througha screen 12 or about150 mesh for the removal of oversize particles. The slurry is thenintroduced into a suitable centrifuge 14 such as, for example, is shownin Patent No- 2,057,156, to separate most of the particlesinerthanlmicrons (plates) from the particles coarser than. 2 microns(stacks). The former fraction is sent t0 a vat 16,l the latter beingsent to an oven 18 where the particles are dried and then stronglyheated (600 to 1000 C.). This expands the individual stacks, suchexpansion being irreversible, and the resulting product is finelygranular, the granules being friable. Complete separation-ofthecoarseand tineparticles inthe-centrifuge 14 is neither practicable nornecessary. A good `prod uct for..theintendedzpurpose can be obtained ifVthe'slurry `ofrco'arse particles-discharged from the centrifugeAcontains up to 30% of fines (i.e., particles smaller than 2 microns).

If a fraction of a more limited range of particle size is desired forcalcining, the coarser fraction from the centrifuge 14 is put through asecond centrifuge for the subdivision and either of the resultingfractions may be sent to the oven 1S.

The calcined stacks are discharged from the oven 18 to a vat 20 wherethey are mixed with a measured proportion of ne particles (plates) fromthe vat 16, the tine particles being conveniently supplied in a slurry.The mixture is then dewatered by filtering or otherwise at 22 and, foreconomical shipping, dried in a suitable drier 24. The resulting productis a novel and valuable ceramic material, the composition andcharacteristics of which can be depended on since the material can beaccurately reproduced. Since the coarser particles (stacks) in thematerial have been already expanded, these particles will not expandfurther when a ceramic article of which they are a part is red. 'Thiseliminates one previously troublesome dimensional change of ceramicarticles together with the resultant internal stresses which have tendedto weaken the ceramic product. I have also found that the pre-calcinedstacks require a substantially smaller proportion of the uncalcined neparticles (plates) as a binder material to weld them together, when aceramic article is fired, for the production of a mechanically strongproduct. A result of this is a substantially smaller shrinkage of theproduct while or after being red. Although the foregoing fact has beenobserved as a result of experimentation, it is not at present understoodwhy precalcined stacks require such a smaller proportion of plates toact as a cementitious matter than do uncalcined stacks for theproduction of strong ceramic ware.

The invention may be carried out by fractionating kaolinitic clay, ashereinbefore described, to Separate the plates (particles of 2 micronsor under) from the stacks (particles of over 2 microns). The morecomplete the separation, the better, but a toleration of up to 30%residual plates remaining with the stacks is permissible. The stacksfraction is calcined at a temperature of, say, l000 C., after which the-calcined stacks are mixed with uncalcined plates in the proportion ofabout 63% to 37%. The resulting mixture is a superior ceramic substancewhich can stand rapid firing with very little shrinkage and unusualfreedom from cracking. It has a high degree of dimensional stability,extraordinary smoothness, and a high fired-modulus of rupture. Thesequalities make the mixture exceptionally well adapted as a raW materialfor the manufacture of excellent art ware.

I claim:

1. In a process of preparing a ceramic raw material, the steps ofremoving from kaolinitic clay most of the particles under 2 micronsequivalent spherical diameter, and calcining the residue at temperaturesfrom 600 to 1000 C.

2. In a process of prepaing a ceramic raw material, the steps ofremoving from kaolinitic clay most of the particles under 2 micronsequivalent spherical diameter, calcining the residue at temperaturesfrom 600 to 1000 C., and mixing with the calcined residue a suiiicientproportion of uncalcined kaolinite particles less than 2 micronsequivalent spherical diameter to bind the mixture when fired.

3. In a process of preparing a ceramic raw material, the steps ofcalcining at a temperature between 600 and 1000 C. kaolinitic clayhaving not over 30% solids content of particles smaller than 2 micronsequivalent spherical diameter, and mixing with the calcined product ameasured proportion of uncalcined kaolinitic particles smaller than 2microns equivalent spherical diameter.

4. A kaolinitic clay calcined at a temperature between 600 and 1000 C.consisting substantially entirely of particles ranging in size from 2microns to 50 microns equivalent spherical diameter.

5. A kaolinitic clay calcined at a temperature between 600 and 1000 C.consisting of at least 70% by weight of particles originally between 2microns and 50 microns equivalent spherical diameter.

6. A ceramic material consisting of kaolinitic particles of originalsize between 2 microns and 50 microns equivalent spherical diameterwhich have been calcined at a temperature between 600 and 1000 C. in thepresence of not over 30% of clay particles smaller than 2 micronsequivalent spherical diameter, mixed with kaolinitic particles smallerthan 2 microns equivalent spherical diameter in a ratio of weightsapproximately 63 to 37.

7. In a process of preparing a ceramic raw material, the steps ofremoving from kaolinitic clay at least of the particles under 2 micronsequivalent spherical diameter, calcining the residue at temperaturesfrom 600 to 1000 C., and mixing with the calcined residue a suflicientproportion of uncalcined kaolinitic particles less than 2 micronsequivalent spherical diameter to bind the large particles when themixture is fired.

8. A ceramic raw material consisting of discrete kaolinitic stackscalcined at av temperature between 600 and 1000 C. and mixed with asulicient poition of raw kaolinitic plates to bind the mixture wheniired.

9. A friable kaolinitic clay product at least 70% of the particles ofwhich are between 2 and 50 microns equivalent spherical diameter andwhich has been calcined at temperatures between 600 and 1000 C.

10. A friable kaolinitic clay product at least 70% of the particles ofwhich are between 2' and 10 microns equivalent spherical diameter andwhich has been calcined at temperatures between 600 and 1000 C.

11. A ceramic raw material consisting of a friable kaolinitic clayconstituent at least 70% of the particles of which are between 2 and 50microns equivalent spherical diameter and which has been calcined attemperatures between 600 and 1000 C., and a second constituentconsisting of raw kaolinitic clay particles less than 2 micronsequivalent spherical diameter.

References Cited in the le of this patent UNITED STATES PATENTS1,446,238 Yumara Feb. 20, 1923 2,041,721 Norton May 26, 1936 2,179,154Lyons Nov. 7, 1939 2,585,116 Gronroos Feb. 12, 1952 2,801,183 KantzerJuly 30, 1957 FOREIGN PATENTS 15,834 Great Britain of 1894 18,911 GreatBritain of 1889 461,837 Great Britain Feb. 27, 1937

6. A CERAMIC MATERIAL CONSISTING OF KAOLINITIC PARTICLES OF ORIGINALSIZE BETWEEN 2 MICRONS AND 50 MICRONS EQUIVALENT SPHERICAL DIAMETERWHICH HAVE BEEN CALCINED AT A TEMPERATURE BETWEEN 600* AND 1000*C. INTHE PRESENCE OF NOT OVER 30% OF CLAY PARTICLES SMALLER THAN 2 MICRONSEQUIVALENT SPHERICAL DIAMETER, MIXED WITH KAOLINITIC PARTICLES SMALLERTHAN 2 MICRONS EQUIVALENT SPHERICAL DIAMETER IN A RATION OF WEIGHTSAPPROXIMATELY 663 TO 37.